1. Field of the Invention
This invention relates to suspension systems for supporting structure on a body, such as supporting the body of a vehicle on a chassis; and more particularly to active vibration isolation suspension systems.
2. Discussion of the Prior Art
Vibration has an adverse affect on the productivity of work vehicles of the type in which an operator cab is supported on a chassis. Such vehicles include agricultural tractors and over the road truck tractors. The vibrations experienced by such vehicles reduce their reliability, increase mechanical fatigue of components, and most importantly increase human fatigue due to acoustic pressure and violent motion of the body.
Therefore, it is desirable to minimize vibration of the vehicle cab in which the operator sits. Through reduction of low frequency vibrations experienced by the cab, driver fatigue and vehicle operability are improved, while reducing higher vibration frequencies decreases human and mechanical fatigue, as well as rattle induced malfunctions.
Previous vehicle cab suspension systems typically performed poorly in the frequency range where the human body is most sensitive, i.e. one to ten hertz. When subjected to vertical movement, or bounce, the human abdomen resonates at approximately four to eight hertz and the head and eyes resonate at ten hertz. The upper torso resonates in response to pitch and roll movement at between one and two hertz. As a consequence, a vehicle suspension system needs high performance at these frequencies and directions to be effective in counteracting vibration.
Very soft cab mounts can provide good attenuation in all directions in this low frequency range (one to ten hertz), but have very poor force rejection ability. In other words, a relatively small external force on the vehicle cab makes it deflect unacceptably. Other cab suspension systems, which are relatively stiff and thus have good force rejection, tend to provide poor low frequency isolation. In many instances, such systems actually amplify the frequencies to which the human body is most sensitive.
It is more desirable to have a suspension system which is hard relative to external forces acting on the cab, but soft to disturbances transferred from the chassis up to the cab, in other words, a hard/soft system. With such a system, the cab feels rigid when the operator climbs into the tractor, but the offending vibrations which would otherwise be transmitted from the chassis to the cab never reach the operator.
A conventional passive suspension system 5 for a vehicle is shown in FIG. 1 and consists of a spring 6 and a damper 7, such as a conventional shock absorber, connected in parallel between the chassis 8 and the body 9 of the vehicle. The motion of the body is defined by the expressions: EQU P=K.delta.+R(V.sub.I -V.sub.0) EQU .delta.=V.sub.I -V.sub.0
where M is the mass of the body, K is the stiffness spring 6, R is the damping coefficient R of the shock absorber, V.sub.0 is the velocity of the body mass and V.sub.I is the inertial velocity of the chassis disturbance. The transmissibility of the suspension is given by: ##EQU1## where s is the Laplace variable.
A trade-off exists in the design of this simple spring and damper suspension system. In order to isolate vibrations at relatively high frequencies, it is desirable to reduce the damping coefficient R. However, such a system tends to resonate, as an automobile with badly worn and ineffective shock absorbers, thereby producing a very springy ride. Increasing the damping coefficient to overcome the springy ride problem decreases the isolation of high frequency vibrations.
A previous attempt to avoid this trade-off, provided a system which dynamically altered the damping force in response to the sensed movement of the mass being isolated. In that system the force Fc exerted by the damper varied in proportion to the mass velocity. Thus the damping coefficient changed in response to the particular disturbance affecting the mass. The motion of the mass in that system is defined by the expressions: EQU P=K.delta.+R.about.V.sub.0 EQU .delta.=V.sub.I -V.sub.0
Thus the dependence on the motion of the chassis has been removed. The transmissibility of the suspension is given by: ##EQU2## As evident from the transmissibility the feedback from the sensed mass motion affects only the damping term in the denominator.